Production of pellets from particulate solid materials



R. PARMELLA Oct. 30, 1962 PRODUCTION OF PELLETS FORM PARTICULATE SOLID MATERIALS Filed Nov. 10, 1959 INVENTOR 77144 PH 7392/ 154 4 A WW M ATTORNEYS United States Patent 3,060,511 PRODUCTION OF PELLETS FROM PARTKCULATE SGLID MATERIALS Ralph Parmella, Norton-on-Tees, England, 'assignor to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain Filed Nov. 10, 1959, Ser. No. 851,969 Claims priority, application Great Britain Nov. 28, 1958 6 Claims. (Cl. 18-475) The present invention relates to the production of pellets from particulate solid materials.

According to the present invention there is provided a process for the production of pellets from particulate solid materials which comprises forcing the particulate material through a die of such proportions that the ratio of the greatest lateral dimension to the length is in the range of 1:5 to 1:15, the said die having been previously conditioned by forcing therethrough particulate material comprising a lubricant.

The particulate material may conveniently be forced through the die by at least one suitably disposed compression means, for example a roller. Preferably a plurality of dies is employed in co-operation with at least one compression means to provide a continuous process. Such a plurality of dies may conveniently be disposed within a carrier which is arranged for relative motion with respect to one or more cooperating rollers. For example the carrier may be stationary and the rollers arranged to move over it, or the carrier may be rotatable and the rollers arranged as idlers.

In a preferred method of carrying out the process of the present invention a rotatable carrier of ring form with a plurality of dies radially disposed therein is employed and one or more idler rollers are positioned to co-operate with the interior surface of the carrier.

It will be understood that the material to be pelleted will be fed by suitable means to the carrier, for example by a vibratory feeder, and that the formed pellets will be detached therefrom by other suitable means, for example suitable knives or ploughs.

The accompanying drawing schematically illustrates equipment suitable for carrying the invention and the mode of operation thereof. FIGURE 1 is an elevational view of a pelletizing apparatus and FIGURE 2 is a fragmentary view of the die of the apparatus of FIG- URE 1.

The material comprising a lubricant with which the die or dies are conditioned may be a mixture of one or more particulate solid materials with the lubricant and may with advantage also comprise an abrasive.

Suitable lubricants for use in the process of the present invention include graphite and talc, also stearic acid and its salts, particularly its alkali salts, e.g. soap.

Abrasives which have been found suitable in the process of the present invention include finely divided silicon carbide and alumina.

The quantities of lubricant and abrasive will depend on various factors, for example on the materials of construction of the dies and on the material to be pelleted. It has been found however that quantities of the order of 2% by weight are in general sufficient.

The process of the present invention may be applied to many materials and has been found particularly suitable for the production of pellets of sizes in the range 0.1 inch to 0.25 inch diameter by 0.05 inch to 0.25 inch long for crystalline urea, employing a die length in the range of 0.75 inch to 1.5 inch.

In general the particulate material to be pelleted should be substantially dry and although a moisture content up to about 1% by weight by be tolerated it is preferable for it not to exceed 0.1%.

In operating the process according to the present invention it has been found that the conditioning of the die or dies by forcing therethrough a particulate material comprising a lubricant, and preferably also, an abrasive, requires a period of two to four hours.

The pelleting of the material per se may then be commenced but it is preferred, particularly on restarting after an interval to force through the die or dies a mixture of a lubricant and some of the material to be finally pelleted, for a period, depending upon the material to be pelleted, to raise the temperature of the dies to an appropriate value before feeding the material to be pelleted per se to the process.

In pelleting crystalline urea it has been found that the appropriate temperature is in the range of C. to 110 C. and preferably about C. and the period required is from 10 to 20 minutes.

Thereafter the pelleting of the crystalline urea per se may be continuous over long periods.

It has also been found that when the process is to be stopped, it is desirable again to feed to the die or dies a material comprising a lubricant in order to ensure the maintenance of the conditioning and satisfactory restarting of the process.

The above-mentioned period of feeding to the die or dies a material comprising a lubricant and the appropriate temperature will vary with the material to be pelleted, but may easily be determined by preliminary trials.

Pellets produced from crystalline urea by the process of the present invention have good mechanical strength and are suitable for many purposes, particularly for use as fertilizers which are to be distributed by mechanical or aerial means. Analysis of the pellets shows there is no substantial increase in the biuret content of the urea.

I claim:

1. A process for pelleting particulate solid material which comprises compacting said particulate material in admixture with a solid lubricant by forcing the mixture through a die orifice in which the greatest lateral dimension to the length is in the range of 1:5 to 1:15, whereby said die is conditioned, removing the thus extruded material and thereafter continuously compacting said particulate solid material free from said lubricant by forcing the same through said die and forming pellets thereof.

2. A process for pelleting crystalline urea having a moisture content less than 2% which comprises compacting an initial quantity of said urea admixed with an amount of the order of 2% of a solid lubricant selected from the class consisting of graphite, talc, stearic acid and its salts by forcing the mixture through a die, in which the greatest lateral dimension to the length is in the range of 1:5 to 1:15 and said length is in the range of 0.75 inch to 1.5 inch, for a period of time suflicient to raise the temperature of said die to within the range 90 C. to C., and thereafter continuously compacting said crystalline urea alone by forcing the same through said die and forming pellets thereof.

3. A process for pelleting crystalline urea as in claim 2 in which said admixture of urea and lubricant contains an amount of the order of 2% of an abrasive and in which said admixture is forced through a plurality of cylindrical dies.

4. A process for pelleting particulate solid material which comprises: compacting said particulate material in admixture with about 2% by weight of a solid lubricant by forcing the mixture through a die orifice in which the greatest lateral dimension to the length is in the range of 1:5 to 1:15 for a period of from two to four hours to condition said die and to raise the temperature of said die; removing the thus extruded material; thereafter continuously compacting said particulate solid material free from said lubricant by forcing the same through said heated die to compact said particulate material; and thereafter forming pellets of said compacted material.

5. A process as in claim 4 in which said admixture also includes about 2% by weight of a finely divided abrasive.

6. A process as in claim 5 in which the lubricant is selected from graphite, talc, stearic acid and its salts and in which the abrasive is selected from the group consisting of silicon carbide and alumina.

References Cited in the file of this patent UNITED STATES PATENTS and. 

